JS temporary dead zone_ Temporary

Hello everyone , I'm wang Jun , An architect who can write code and recite poetry . Let's talk about it today js Temporary dead zone _ Temporary , I hope it can help you make progress !!!

stay Js What is temporality in ?

 The temporary dead zone is for 'const','let' The concept of these two keywords .

 First, the variable raises this js The basic concept of ,'const' and 'let' As a block level scope, it cannot be avoided .

 and 'var' Different , These two keywords limit the scope to ‘ block ’ in , In this block , The variables defined by these two keywords have been allocated memory .

 That is, in fact ' There is ' 了 , But when the program is not executed to the declaration , Accessing this variable will report a reference error .

 This is the time , For this variable, it is ' Temporary dead zone ', Generally speaking, this variable exists , But it doesn't exist completely 

Copy

Code up

 This code is created by Java Architects must see the net - Structure Sorting 
// The situation a 、
if (true) { 
   
  // TDZ Start 
  tmp = 'abc'; // ReferenceError
  console.log(tmp); // ReferenceError


  let tmp; // TDZ end 
  console.log(tmp); // undefined


  tmp = 123;
  console.log(tmp); // 123
}

Copy

In the above code , stay let Command declaration variable tmp Before , All variables tmp Of “ dead zone ”.

// Scenario two 
var tmp = 123;
if (true) { 
   
  tmp = 'abc'; // ReferenceError
  let tmp;
}

Copy

In the above code , There are global variables tmp, But in block level scope let A local variable is declared tmp, Causes the latter to bind the block level scope , So in let Before declaring variables , Yes tmp The assignment will report an error .

ES6 Make it clear , If a block exists let and const command , Variables declared by this block for these commands , Closed scopes have been formed from the beginning . Always use these variables before declaring them , You're going to report a mistake .

 This code is created by Java Architects must see the net - Structure Sorting 
// Scenario three 
//  Don't complain 
var x = x;
//  Report errors 
let x = x;
// ReferenceError: x is not defined

Copy

The above code reports an error , It's also because of the temporary dead zone . Use let When variables are declared , As long as the variable is used before the declaration is complete , You're going to report a mistake . This is the case with the above line , In variables x Before the execution of the declaration statement is completed , Just go and get it x Value , Result in an error ”x Undefined “

That's the end of today's article , Thank you for reading ,Java Architects must see I wish you a promotion and a raise , Good luck every year .